1. Technical Field
The present disclosure relates to signal-generating circuits that generate high-frequency signals.
2. Description of the Related Art
In wireless communication devices, PLL frequency synthesizers are primarily used as signal-generating circuits for generating desired high-frequency signals. Voltage-controlled oscillators (VCOs) are widely used in PLL frequency synthesizers. Voltage-controlled oscillators have a narrow variable frequency range and are thus hard to employ when a broad variable frequency range is required or when a high frequency is used. In high-speed wireless communication (e.g., IEEE 802.11ad) that uses a 60 GHz band, or millimeter waves, for example, four channels at 58.32 GHz, 60.48 GHz, 62.64 GHz, and 64.80 GHz are assigned within a 9 GHz band ranging from 57 GHz to 66 GHz, and it is thus difficult to handle all four channels.
Addressing such an issue, U.S. Pat. No. 8,229,352 discloses a technique that enables a voltage-controlled oscillator to handle the four frequencies required in the 60 GHz band by fixing an oscillation frequency of the voltage-controlled oscillator.